Liquid phase polymerization processes can be grouped into two general types: bulk processes and slurry processes. Both the bulk process and the slurry process are described by Ser van der Ven in “Polypropylene and Other Polyolefins”, © 1990, Elsevier Science Publishing Company, Inc., pp. 119-125. Both the slurry process and the bulk process are described as operating with a suspension of growing polymer particles in a liquid. In the case of the slurry process, the liquid is an inert solvent, such as hexane. In the case of the bulk process it is liquid monomer.
U.S. Pat. No. 4,522,994 to Chiba, et al, describes highly crystalline, high melt flow rate polypropylene produced using a Ziegler-Natta (ZN) catalyst in a slurry process. In order, however, to isolate the desired highly crystalline, high melt flow rate polymer, Chiba was obliged to extract significant amounts of soluble polymer produced during the polymerization reaction. Thus, although Chiba's insoluble polymer is highly crystalline, the process described in 4,522,994 does not control the quantity of soluble polymer produced. This lack of control results in reduced yields of the desired product and significant waste.
Another means of achieving a high melt flow rate in a polymer is through post polymerization treatment with a vis-breaking agent to reduce the average molecular weight of the polymer. This method, however, necessitates an additional process step and negatively affects the molecular weight distribution (Mw/Mn) of the polymer. Vis-breaking also creates additional xylene soluble material, which negatively affects the properties of the desired material.
U.S. Pat. No. 6,323,150 to Kojoh, et al, describes ZN catalysts employing a combination of two internal donors selected from a polycarboxylic acid compound and a polyether. The catalysts described in U.S. Pat. No. 6,323,150 are described as being capable of producing olefin polymers of high stereoregularity. All of the examples provided in U.S. Pat. No. 6,323,150, however, describe polymers produced using a slurry process.